Descartes argues that we know that our bodies exist because something must be causing our impressions of them–i.e., we aren’t simply dreaming but perceived reality is actually real. Spinoza, however, disagrees. He argues instead that bodies do not affect the mind any more than the mind affects the body. Rather, Spinoza aruges, ideas generate ideas and bodies construct other bodies.
For Spinoza, Descartes is wrong, because not only has he privileged the discrete and finite individual subject over the infinite Substance, God, but Descartes has also privileged waking over dreaming. Mechanist that he is, Descartes assumes that cause only works one one way, and in one direction: physical causes lead to affects. But what if, as Spinoza says, affect, emotion or feeling, is (at least logically, or formally) prior to (mechanical) bodily sensation? What if we experienced affects, or passions, not because bodies have moved us, but rather because prior affects had affected us? This would be a radically different view of the world, one in which not Reason but Desire, a continuous chain of affects, were the actual source of reality.
In the Cartesian epistemology, Time is merely the linear passage of identical moments, each of them merely ticking by with clockwork regularity. In Spinoza’s ontology, on the other hand, Time rather functions in an “epicentric” (or catastrophic) manner, dynamically gathering up the past into a living reality. According to this model, the truth of Humanity, our essential nature, is not Reason but Appetite (cf. The Ethics, Part III, Prop. IX, note.) For Spinoza it is not “I think therefore I am;” but rather “I eat therefore I am;” or “I consume, therefore I am.” Consciousness, in parallel with the Body, always plunges forward, courses and surges through the world, gathers itself up and sustains itself while pulsing into the future. According to this reading of Spinoza (one which makes him appear akin to Schopenhauer), the Will craves more life, more experience, and then conjures up the appearance of images to gratify that desire, or to fulfill that wish, and thereby sustain itself. The ego prolongs its own identify by perpetuating a chain of fantasies.
This is really only another way of saying that our normal state of consciousness is not waking but dreaming. To “wake” is to lapse back into the Cartesian world, which, for Spinoza, in fact constitutes a return to dogmatic slumber. See, for instance, Spinoza’s Ethics, the extended note to Part III, prop. II (pg. 133 in the Dover edition):
“But, it will be urged, it is impossible that solely from the laws of nature considered as extended substance [i.e., without subjective intension], we should be able to deduce the causes of buildings, pictures, and things of that kind, which are produced only by human art; nor would the human body, unless it were determined and led by the mind, be capable of building a single temple. However, I have just pointed out that the objectors cannot fix the limits of the body’s power, or say what can be concluded from a consideration of its sole nature, whereas they have experience of many things being accomplished solely by the laws of nature, which they would never have believed possible except under the the direction of the mind: such are the actions performed by somnambulists while asleep, and wondered at by their performers when awake.”